The present invention relates generally to the field of instantaneous frequency measurement receivers with digital processing, and more particularly to an angle-of-arrival (AOA) application of a digital channelized IFM receiver.
The conventional IFM receiver is a radio frequency receiver used primarily in electronic warfare. Its basic function is to measure the frequency of pulsed signals radiated from hostile radar. Generally, it may be said that IFM receivers measure the frequencies of incoming RF signals utilizing interferometric techniques by detecting the phase shift magnitudes produced in multiple, calibrated delay lines. For instance, the received RF signal is divided and simultaneously introduced into a non-delayed path and a delay line of known length. Since the delay and non-delayed receiver paths are functions of the input signal frequency, conversion of the phase difference signals to video signals provides signals whose amplitudes are related to phase delay. These signals typically take the form of sin .omega..tau. or cos .omega..tau., where .omega. is the angular frequency or the processed input signal, and .tau. is the known time delay. The sin .omega..tau. and cos .omega..tau. signals are delivered to an encoding network which makes amplitude comparisons of the signals, determines the numerical value of .omega., and generates the digital frequency descriptive word.
An IFM receiver has many attractive features necessary for electronic warfare applications, such as small size, light weight, wide instantaneous bandwidth, and fine frequency resolution.
In a digital receiver, the incident radiation is mixed with a local oscillator signal and down converted to an intermediate frequency (IF). This IF signal is discretely sampled and further processing is done using digital techniques. The frequency of the incident radiation may be determined by performing a discrete Fourier transform on the sampled signal.
The following United States patents are of interest.
U.S. Pat. No. 5,280,288--Sherry et al PA1 U.S. Pat. No. 5,262,837--Shyy PA1 U.S. Pat. No. 5,235,287--Sanderson et al PA1 U.S. Pat. No. 5,198,748--Tsui et al PA1 U.S. Pat. No. 5,102,219--Skagerlund PA1 U.S. Pat. No. 4,996,533--May et al
The patent to May et al discloses a method for mapping ocean currents using a linear array of antennas each with its own receiver/digitizer system. The summing and phasing of the signals are done using two successive complex Fourier transforms. The patents to Sanderson et al and Tsui et al disclose the use of sample signals being further transformed and the phase and amplitudes calculated. The patent to Sherry et al discloses the use of Fast Fourier Transform in target detection systems. The patents to Skagerlund and Shyy disclose digitization of the received signals.